There is also another type that is 16.6 volts as well that I could get. I thought about running two 7.2 in series but found these other one's
So why bother with two battery's? or does running them like that give you a better option?

Another Question as I am still confused a bit LOL
Like the above 19.8 volt $$$ and these below are a bit cheaper

This starter I am getting is a 6.1 geared ratio. from 12 to 18 volts.

So I am also looking at the LiFe 4200 MAH @ 30C @ 13.2 volts
and also the LiFe 4200 MAH @ 30 C @ 16.5 volts.
So what Will the diff be between all 3 of these for a days use? or power ?
Would I be just fine with the lesser volts?
Or maybe the better thing to do, is ask what one would you use?

The higher voltage you supply to a DC motor the faster it spins and the more current drawn. More current less battery life. All your batteries have the same capacity. Seriously - anything you've listed should work. I'd use the highest "C" rated batteries with the highest capacity that will physically fit and stay within the voltage rating of the motor. I never like to push voltage limits.
John

Also, remember that the nominal voltage for a lipo (3.7V per cell) is when it's almost outa gas. You should really make your choice based on the fully charged voltage. A 4S pack is 16.8 fully charged, so a good choice for this application, a 6S pack would be 25.2V! Spin baby spin!!! Also, since that's a geared starter, you may even be good on 3S.
As John said, the lower the voltage that's able to do the job, the longer your starter will last.
Cheers,
Pete